Many integrated circuit microprocessors are adapted to enter a "sleep mode" in order to reduce power consumption when processing is not needed. Such processors are adapted to execute instructions, variously called WAIT, STOP or SLEEP instructions, which may be included in software routines to suspend power-consuming functions within the processor. Typically, those portions of the microprocessor which are cycling at high clock rates are totally disabled to drastically reduce the power consumed by the device. In some processors, different instructions may be available to the programmer to disable the processor to different degrees; for example, a STOP instruction may completely disable both the processor and the internal clock, whereas a WAIT instruction may retain selected internal functions, such as an interval timer for generating a hardware interrupt which reawakens the processor after a predetermined period of inactivity.
As used in this specification, the term "sleep mode" is intended to refer to any such power conservation mode which renders the processor temporarily unable to execute instructions in the usual way. To permit a microprocessor which has been placed in sleep mode to promptly resume normal operations, the contents of internal data and address registers are saved. Microprocessors having these characteristics are available from several manufacturers and include the MC68HC11 family of microcontrollers manufactured by Motorola Inc., Oak Hill, Tex., which are described in detail in Volume II of Motorola Microprocessor, Microcontroller and Peripheral Data, Motorola Inc. (1988).
Microprocessors of this class have been used extensively to implement battery powered systems, such as automotive electronic controls, which preferably consume a minimum amount of power when the controls are not in use. In automotive systems, processors used for control purposes may be placed in their sleep mode when the automobile's ignition key is turned off. By way of example, U.S. Pat. No. 4,698,748 describes an automotive control system employing a processor which enters a sleep mode to conserve power with the ignition key is off.